Flush mounted coaxial horn antenna



p 1963 R. w. KREUTEL. JR 3,400,404

FLUSH MOUNTED COAXIAL HORN ANTENNA Filed July 20. 1965 p or art INVENTOR RANDALL W. KREUTEL JR.

ATTORNEY United States Patent 3,400,404 FLUSH MOUNTED COAXIAL HORN ANTENNA Randall W. Kreutel, In, Walpole, Mass., assiguor to Sylvania Electric Products Inc., a corporation of Delaware Filed July 20, 1965, Ser. No. 473,346

5 Claims. (Cl. 343784) ABSTRACT OF THE DISCLOSURE A hollow cylinder having an inner conductive surface is employed in conjunction with a mode rejection filter mounted coaxially within the cylinder to form-a horn antenna.

This invention relates to antennas and more particularly to flush mounted coaxial horn antennas especially suited to electrically steered arrays.

In electrically steered antenna arrays, the antenna elements comprising the array must be electrically small in order that a sufiicient number of elements can be included within the space of a wavelength to give the desired radiation pattern. An antenna element that has been employed in such arrays is the coaxial horn which operates in the TE mode. A major problem of such antennas, however, is that undesired modes are generated which, in turn, cause a reduction in the usable bandwidth over which the antenna can be operated. In addition, impedance matching between the antenna and free space has heretofore been accomplished with a protruding transition section which extends out from the antenna aperture. Such a protrusion is mechanically unattractive, especially in large arrays which may contain thousands of antenna elements, since it is more susceptible to damage by external forces than a flush surface.

It would be desirable, and it is an object of the invention, to provide a flush mounted coaxial horn antenna which does not generate undesired modes of radiation, which has an improved bandwidth, and which is matched to free space without the necessity of protruding matching sections.

In accordance with the present invention a periodically loaded coaxial waveguide is employed as a mode rejection filter which supports the TE mode and rejects the TEM mode, and which'also provides a relatively high impedance to match that of free space.

The invention will be more fully understood from the following detailed description, taken in conjunction with the drawing, in which:

FIG. 1 is a partly cut away elevation view of a conventional coaxial horn antenna; and

FIG. 2 is a partly cut away pictorial View of an antenna according to the invention.

A conventional coaxial horn antenna is illustrated in FIG. 1, and includes an outer conductive cylinder connected to a ground plane 12 which extends at right angles to cylinder 10, and an inner conductive cylinder 14 which terminates on one end in an impedance matching section 16 which extends beyond the antenna aperture 18. A dielectric dome 20, which follows the contour of section 16 is provided to cover the aperture. The antenna is energized by a balanced feed connected to the inner and outer conductors or coupled to the axial cavity by probes or other well known means.

The protrusion of impedance matching section 16 and array. A further disadvantage is the generationof the TEM mode by the uniform coaxial structure, which degrades the desired radiation pattern and reduces the usable bandwidth of the array. Even though the antenna can be designed to minimize TEM mode excitation, mutual coupling between antenna elements in an array can cause generation of this mode.

The antenna design illustrated in FIG. 2 overcomes the above-discussed problems by employing a periodically loaded waveguide which rejects the TEM mode and matches the free space impedance. The periodically loaded structure is supported within outer cylinder 40 on support posts 44, andincludes alternate low impedance sections, 34 and 36 which are above cutoff for the TE mode, and high impedance sections 30 and 32 which are below cutoff for the TE mode. The composite structure is above cutoff in the design band. Sections 32, 34 and 36 dome 20 beyond the antenna aperture is mechanically discomprise a mode rejection filter for the TEM mode and a pass filter for the TE mode, and also provide impedance matching between the low impedance input line 38 and high imepdance output line 30. The impedance of line 30 is designed to balance out the reactance of the aperture thereby to match the free space imepdance. A dielectric cover 42 is provided in the aperture to support the. waveguide structure and protect it from the environment. The antenna is energized by a well known balanced feed to launch the TE mode, or can be energized by a suitable unbalanced feed which does not excite the TEM mode. A suitable ground plane, not shown, is provided as in the antenna of FIG. 1.

As shown in FIG. 2, the overall diameter is .457 A and 34 and 36 are, respectively, .330 i and .362 A in diameter, and .220 )t and .132 A in length. High impedance sections 30 and 32 have a diameter of .165 A and are .275 A and .110 A in length, respectively. The wavelength t is the TE wavelength at the center of the operating band.

The dimensions shown in FIG. 2 are representational and can be varied to suit the particular operating requirements. The diameter of section 36 can be the same as the diameter of section 34, but is slightly larger in the illustrated embodiment to provide shunt compensation which improves the impedance match. In an antenna that was constructed for operation in the L band, radiation in the T13 mode was achieved over a 1.4 to 1 bandwidth without significant generation of the unwanted TEM mode. It has been experimentally determined that this antenna design has a broader and less frequency sensitive beamwidth and better polarization quality than the radiation pattern of an antenna of conventional design, such as the type illustrated in FIG. 1. Moreover, the patterns of an array comprised of this antenna are much improved over conventional arrays due to the absence of undesired modes and better mutual coupling characteristics between elements.

From the foregoing it is evident that a flush mounted horn antenna has been provided which does not excite unwanted modes, which is matched to free space without transition sections extending beyond the aperture, 'and which is especially suited to use in an array. The invention is not to be limited by what has been particularly shown and described, except as indicated in the appended claims.

What is claimed is:

1. A flush mounted coaxial horn antenna comprising, a uniform cross sectioned outer conductor having an input end and an output end, and a periodically loaded mode rejection conductor concentrically disposed within said outer conductor, said inner conductor including alternate sections of high and low impedance with a high impedance section adjacent said output end and a low impedance section adjacent said input end, said low impedance input end being matched to the high impedance of free space by the cooperation of said high impedance section and said outer conductor.

2. A flush mounted coaxial horn antenna comprising, a cylindrical outer conductor, and a periodically loaded inner conductor coaxially disposed within said outer conductor, said inner conductor including in the order named a first section adjacent the antenna aperture .275 A long and .16 5 t in diameter, a second section .220 A long and .330 A in diameter, a third section .110 A long and .165 A in diameter, a fourth section .132 A and .362 x, in diameter.

3. A flush mounted coaxial horn antenna comprising: a hollow cylinder having an inner conductive surface;

and a periodically loaded mode rejection structure coaxially mounted entirely Within said cylinder, said structure having a high impedance radiating end and a feed end, said radiating end cooperating with said the impedance of the outside media.

4. A flush mounted coaxial horn antenna according to claim 3 wherein said hollow cylinder has a uniform cross section along the longitudinal axis and wherein said mode rejection structure is a TEM mode rejection structure.

5. A flush mounted coaxial horn antenna according to claim 3 further including a dielectric aperture plate disposed on the end of said cylinder at the radiating end of said mode rejection structure.

References Cited UNITED STATES PATENTS 2,403,909 7/ 1946 Carter 343-786 2,577,510 12/1951 Cohn 33373 2,928,092 3/ 1960 Hatch 343784 3,205,498 9/1965 Child 343705 ELI LIEBERMAN, Primary Examiner. 

